In a height adjustment mechanism for a vehicle seat, a clutch unit is used which is configured such that an output shaft is rotated by an operation of turning up and down a lever provided at an input side thereof but is not rotated even when a force which is derived from the weights of the seat and an occupant and by which the seat is biased downward is applied to the output shaft.
In such a clutch unit, a ratchet mechanism configured to transmit a driving force to the output shaft when the lever is moved from a neutral position upward or downward but not to transmit the driving force when the lever is moved from an upper or lower position back to the neutral position, and a brake mechanism configured to transmit an input torque received from the ratchet mechanism to the output shaft but not to transmit a reverse input torque received from the output shaft to the ratchet mechanism are provided. In the clutch unit, a return spring for returning the lever to the neutral position is provided (Patent Literatures 1, 2).
Further, in the brake mechanism, an outer race having a cylindrical inner peripheral surface and a cam having an outer peripheral surface nonparallel to this inner peripheral surface are provided, and rollers are arranged between the inner peripheral surface of the outer race and the outer peripheral surface of the cam. Spaces between the inner peripheral surface of the outer race and the outer peripheral surface of the cam provide wedge-like spaces each shaped to be narrower toward outer sides in the circumferential direction; the rollers as held fast between the outer peripheral surface and the inner peripheral surface serves to shut off the transmission of the reverse input torque received from the output shaft. In order to release this brake, a transmission member (e.g., connection member 15 in FIG. 8 of Patent Literature 2) configured to push the rollers in the circumferential directions while engaging with the cam is employed.